The present invention relates to electric machines.
There is an increasing demand for compact and efficient electric machines. One factor influencing the efficiency of an electric machine is the winding density within the stator slots of the electric machines. An electric machine having a relatively high winding density, often referred to as a high slot fill factor, will generally be more efficient.
One method for enhancing the density of the stator windings is to use hairpin conductors which have a pair of legs that are inserted into the slots of the stator core. The free ends of the hairpin conductors are then welded to adjacent hairpin conductors to define a stator winding. The use of such hairpin conductors having a rectangular cross section can be used to achieve a relatively high slot fill factor.
The use of hairpin conductors, however, can be complex to manufacture, requiring the individual hairpin conductors to be inserted into slots and formed into the proper shape. Most significantly, the use of hairpin conductors also results in the need to weld each of the individual legs of the hairpin conductors to adjacent conductors to form a continuous stator winding. To reduce manufacturing costs, the cross sectional size of the hairpin conductors can be increased to reduce the number of conductors and thereby reduce the number of welds and assembly requirements.
While increasing the cross sectional size of the individual conductors reduces the number of welds, it may also have unwanted consequences. More specifically, when the electric machine is operating at a high speed and generating an oscillating, high frequency, AC current, the conductors are subjected to what is commonly referred to as the skin effect with only that portion of the conductor near the outer surface of the conductor conducting electrical current. In such circumstances, the center portion of the conductor does not conduct any meaningful electrical current. The effect is sometimes referred to as “AC copper loss” because when the conductor is formed out of copper, the copper material in the center of the conductor is not effective in conducting electrical current.
An electric machine design which reduces such copper losses while also being capable of being manufactured in a cost efficient manner is desirable.